Adenosine is a naturally occurring purine nucleoside, from which is derived a range of agonists at adenosine receptors having considerable potential in the treatment of human disease (Life Sciences, 1991, 49, 1435-1453; Journal of Medicinal Chemistry, 1992, 35, 407-422; Annual Reports in Medicinal Chemistry, 1993, 28, 295-304).
Adenosine has been shown to have a number of significant effects on the mammalian central nervous system (CNS) (Annual Reports in Medicinal Chemistry, 1988, 23, 39-48; Adenosine in the Nervous System, T. W. Stone, Ed., Academic Press Ltd., London 1991) especially under conditions of neuronal stress where the compound appears to act as an endogenous neuroprotectant (Progress in Neurobiology, 1988, 31, 85-108, Trends in Pharmacological Sciences, 1992, 11, 439-445). For example, the concentration of adenosine has been demonstrated to rise greatly in certain brain regions following epileptic seizures or conditions of neuronal ischaemia/anoxia (Brain Research, 1990, 516, 248-256).
It has been established for some years now that centrally acting adenosine receptor agonists or compounds which increase extracellular adenosine levels can exhibit what is termed neuromodulator activity (Trends in Neurosciences, 1984, 164-168). Such substances influence (Trends in Neurosciences, 1984, 164-168). Such substances influence the release of neurotransmitters in regions of the central nervous system (Annual Review of Neuroscience, 1985, 8, 103-124; Trends in Neurosciences, 1984, 164-168), with particular inhibitory effects on the release of the excitatory amino acid glutamic acid (glutamate) in the CNS (Nature, 1985, 316, 148-150) especially under ischaemic conditions (Journal of Neurochemistry, 1992, 58, 1683-1690).
There are several CNS ailments for which this adenosine receptor mediated neuromodulator activity is accepted by persons skilled in the art as being of clear therapeutic benefit including the treatment of convulsive disorders (European Journal of Pharmacology, 1991, 195, 261-265; Journal of Pharmacology and Experimental Therapeutics, 1982, 220, 70-76; European Journal of Pharmacology, 1993, 242, 221-228), prevention of neurodegeneration under conditions of brain anoxia/ischaemia (Neuroscience Letters, 1987, 83, 287-293; Stroke, 1988, 19, 1133-1139; Neuroscience, 1989, 30, 451-462; Pharmacology of Cerebral Ischaemia 1990, (Kriegelstein, J. and Oberpichler, H., Eds., Wissenschaftliche Verlagsgesellschaft mbH: Stuttgart, 1990, pp 439-448; Trends in Pharmacological Sciences 1992, 11, 439-445) or the use of a purinergic agent in the treatment of pain (European Journal of Pharmacology, 1989, 162, 365-369; Neuroscience Letters, 1991, 121, 267-270).
Adenosine receptors represent a subclass (P.sub.1) of the group of purine nucleotide and nucleoside receptors known as purinoreceptors. This sub-class has been further classified into distinct receptor types which have become known as A.sub.1, A.sub.2 and A.sub.3. Extensive research has been carried out in a quest to identify selective ligands at these sites. Selective ligands exist for A.sub.1, A.sub.2 and A.sub.3 adenosine receptors and the structure-activity relationships of the various reference ligands have been reviewed (Comprehensive Medicinal Chemistry, Volume 3, (Hansch, C., Sammes, P. G. and Taylor, J. B., Eds., Pergamon Press PLC: 1990, pp 601-642, Journal of Medicinal Chemistry, 1994, 37, 636-646). Among the known adenosine receptor agonists most selective for the A.sub.1 receptor over the A.sub.2 receptor are the examples where the adenine nucleus is substituted with a cycloalkyl group on the amino function, for example N-cyclopentyladenosine (CPA) and N-cyclohexyladenosine (CHA) (Journal of Medicinal Chemistry, 1985, 28, 1383-1384) or 2-chloro-N-cyclopentyladenosine (CCPA) (Naunyn-Schmiedeberg's Arch. Pharmacol. 1988, 337, 687-689).
There is evidence for further subdivision of adenosine receptors into the subtypes A.sub.2a, A.sub.2b (of high and low affinity) A.sub.3 and A.sub.4. The latest status of these subtypes has been reviewed (Drug Development Research, 1993, 28, 207-213; Trends in Pharmacological Sciences 1993, 290-291; Pharmacological Reviews, 1994, 46, 143-156). The A.sub.3 receptor (Proceedings of the National Academy of Sciences of the USA, 1992, 89, 7432-7436; Trends in Pharmacological Sciences, 1994, 15, 298-306) appears to be responsible for some of the cardiovascular effects of reference ligands (British Journal of Pharmacology, 1993, 109, 3-5).
Various examples of N-heteroarylalkyl substituted A.sub.1 selective adenosine analogues have been reported in the literature. It should be noted that some of these are named as N.sup.6 -substituted adenosine derivatives, but this is equivalent to ACS-approved nomenclature where compounds substituted on adenosine's 6-amino position are referred to as N-substituted adenosine derivatives. Derivatives of adenosine with the heteroatoms sulphur, oxygen or nitrogen bonded directly to the 6-amino substituent are not common in the chemical literature, but those cases known are summarised below.
Derivatives with hydrogen at the purine 2-position include N-aminoadenosine, N-[(N-methyl-N-phenyl)amino]adenosine, N-hydroxyadenosine, N-methoxyadenosine and N-benzyloxyadenosine (Journal of Medicinal Chemistry, 1985, 28, 1636-1643); N-ethoxyadenosine (Chemical and Pharmaceutical Bulletin, 1973, 21, 1676-1682; ibid., 1973, 21, 1835-1838); N-(methylamino) adenosine and N-[(N-hydroxy-N-methyl)amino]adenosine (Journal of Medicinal Chemistry, 1968, 11, 521-523). A range of compounds which have no further substitution on the ribose moiety have been published by Novo Nordisk (Bioorganic and Medicinal Chemistry Letters, 1993, 3, 2661-2666).
Examples of adenosine derivatives with oxygen or nitrogen atoms bonded to the 6-amino substituent, containing an additional purine 2-substituent are 2-amino-N-hydroxyadenosine (Journal of Medicinal Chemistry, 1972, 15, 387-390); 2-amino-N-aminoadenosine (Chemical and Pharmaceutical Bulletin, 1969, 17, 2373-2376); 2-amino-N-methoxyadenosine (Chemical and Pharmaceutical Bulletin, 1975, 23, 464-466); 2-chloro-N-hydroxyadenosine (Journal of Medicinal Chemistry, 1991, 34, 2226-2230), 2-fluoro-N-hydroxyadenosine and 2-fluoro-N-aminoadenosine (Journal of Medicinal Chemistry, 1970, 13, 427-430) and 2-fluoro-N-methoxyadenosine (Journal of Medicinal Chemistry, 1971, 14, 816-819). These articles involve compounds with intact ribose moieties.
In the above scientific articles, no mention is made of any pharmacological effects of the compounds concerned on the central nervous system.
There are also very few examples of compounds designed as adenosine receptor agonists where the ribose moiety in adenosine is chemically modified, and many of those known have poor affinity for the adenosine receptor (Journal of Medicinal Chemistry, 1986, 29, 346-353). However, minor modifications at the 3'- and 5'-positions appear to be allowed and amongst these the 5'-chloro-5'-deoxy adenosines show particularly good receptor affinity (Journal of Medicinal Chemistry, 1989, 32, 8-11). Other scientific articles also describe 5'-modifications of adenosine derivatives (Journal of Medicinal Chemistry, 1986, 29, 1683-1689).
EP Publications No. 181,128 and 181,129 disclose 5'-deoxy adenosine derivatives containing 5'-hydrogen, 5'-halogen and 5'-methylthio, which are claimed to have desirable antiinflammatory, analgesic as well as CNS and antihypertensive properties respectively. EP Publication No. 232,813 discloses N-substituted adenosines including a larger range of 5'-modified compounds which are also claimed to have desirable CNS and antihypertensive properties. PCT Publication WO 94/02497 reveals certain sulphohydrocarbon derivatives of adenosine, where the possibility exists for substitution at the 5'-position of the ribose moiety. In PCT Publication WO 88/03147 5'-substituted adenosine derivatives with selectivity for the adenosine A2 receptor are disclosed.
In U.S. Pat. No. 4,962,194 methods for preparing 5', N-disubstituted adenosine derivatives are revealed. GB Patent No. 1,101,108 discloses 5', N-disubstituted adenosine analogues which possess cardiovascular activity. U.S. Pat. No. 3,910,885 reveals 4'-alkoxy and 4'-haloalkoxy nucleosides. PCT Publication WO 94/06348 discloses a number of pyrrolo[3,4-d]pyrimidine structures which are formally isosteric with adenosine and which are modified with substituents at the sugar 5'-position. U.S. Pat. No. 5308837 covers the use of 5'-amine substituted adenosine analogues as immunosuppressants.
In U.S. Pat. No. 3,819,613, substituted adenosine analogues with hydrazone derivatives on the 6-amino function are disclosed as hypotensive agents. In GB Patent No. 1,351,501, adenosine and 2-aminoadenosine derivatives having a --NH--R.sub.2 group joined to the 6-amino function are disclosed as coronary dilators and platelet aggregation inhibitors. In EP Publication No. 152,944, a series of 2-, 6- and 8-substituted adenosine derivatives are described having activity as antiallergy agents. In EP Publication No. 253,962, adenosine and 2-haloadenosine analogues having an alkyl, cycloalkyl or an aralkyl group attached to the 6-amino function are described with activity as antidementia agents.
In EP Publication No. 402,752, derivatives of adenosine unsubstituted in the 2-position are described which have a substituted heteroaromatic 1-pyrrolyl moiety attached to the 6-amino group. In PCT Publication No. WO 91/04032, methods of preventing neural tissue damage in neurodegenerative diseases by increasing extracellular concentrations of adenosine are described. Examples are given of prodrug esters of AICA riboside which are claimed to be centrally acting neuroprotective agents. In PCT Publication No. WO 92/02214, analogues of AICA riboside are described which increase extracellular adenosine levels with beneficial effects claimed in peripheral and CNS ischaemia. In PCT Publication No. WO 90/05526, 2-(alkylalkynyl)adenosine derivatives are described for treatment of ischaemic disease of the heart and brain. In EP Publication No. 423 777 a method for treating gastrointestinal motility disorders using N(6) (substituted aminoalkyl) adenosine derivatives is disclosed. EP Publication No. 490 818 describes a new use of 2'-O-methyl adenosine derivatives for a range of ailments including neurodegenerative disorders.
The present invention relates to new adenosine analogues with modified ribose moieties which show potent binding in vitro to the adenosine A1 receptor, and which also display selectivity for A.sub.1 receptor binding in vitro over that to the A.sub.2 receptor subtype. In addition, the compounds contained in this invention have a relatively high lipophilicity, especially when compared to adenosine analogues which are not substituted on the 6-amino group or the purine 2-position. This latter property makes these compounds suitable for passage across the blood brain barrier.
The compounds are also substrates for nucleoside-specific active transport systems into the CNS across the blood barrier. These useful properties support the notion that the compounds are candidate drugs for treatment of the CNS ailments mentioned within this invention in humans as well as cardiovascular disorders such as cardiac ischaemia.
The compounds of the invention are purine derivatives of formula I, or a pharmaceutically acceptable salt thereof: ##STR2## wherein X is halogen, amino, perhalomethyl, cyano, C.sub.1-6 -alkoxy, C.sub.1-6 -alkylthio or C.sub.1-6 -alkylamino;
A is methyl, halomethyl, cyanomethyl, aminomethyl, vinyl, methylthiomethyl or methoxymethyl; PA1 R.sup.1 is selected from the groups consisting of ##STR3## wherein Q is nitrogen or carbon, n is 1 to 3 and where the group (a) may be optionally substituted with one or two C.sub.1-6 -alkyl groups, C.sub.2-6 -alkenyl, C.sub.2-6 -alkynyl, phenoxy, phenylsulphonyl, phenylsulphinyl, phenylthio, hydroxy, phenyl, C.sub.1-6 -alkoxy or C.sub.1-6 -alkoxy-C.sub.1-6 -alkyl, phenylthioalkyl or ##STR4## wherein Y is O, S or NZ, where Z is H, C.sub.1-6 -alkyl or phenyl, and where the group (b) may be optionally substituted with C.sub.1-6 -alkyl, C.sub.2-6 -alkenyl, C.sub.2-6 -alkynyl, phenoxy, phenyl, C.sub.1-6 -alkoxy or C.sub.1-6 -alkoxy-C.sub.1-6 -alkyl, or PA1 R.sup.1 is --NR.sup.2 R.sup.3 or --YR.sup.4, wherein Y is oxygen; PA1 R.sup.2 is C.sub.1-6 -alkyl; PA1 R.sup.3 is phenyl or C.sub.1-6 -alkyl which may be substituted by phenyl or phenoxy; PA1 R.sup.4 is C.sub.1-6 -alkyl or C.sub.3-8 -cycloalkyl, which may be substituted by phenyl or phenoxy. PA1 Active compound 5.0 mg PA1 Lactosum 67.0 mg Ph. Eur. PA1 Avicel.TM. 31.4 mg PA1 Amberlite.TM.IRP 88 1.0 mg PA1 Magnesii stearas 0.25 mg Ph. Eur.
In certain examples, the group R.sup.1 can contain one or more asymmetric carbon atoms in addition to those asymmetric centres already present in the molecule. In examples where this is the case, this invention includes all resulting diastereoisomers and mixtures thereof.
Various salts of compounds of formula (I) can be prepared which is physiologically acceptable. These include addition salts derived from inorganic or organic acids, for example, acetates, fumarates, glutarates, glutaconates, lactates, maleates, methanesulphonates, phosphates, salicylates, succinates, sulphates, sulphamates, tartrates and paratoluenesulphonates. In some cases, solvates of either the free nucleosides or the acid addition salts can be isolated and these solvates may, for example, be hydrates or alcoholates.
Compounds of formula (I), which act as adenosine receptor agonists, are useful in the treatment of central nervous system conditions such as anxiety, neuronal ischaemia/anoxia, convulsive disorders (epilepsy) and neurodegeneration (including Parkinson's disease) in humans. This includes treating disorders where the blood flow to the brain is interrupted, for example during traumatic head injury, cardiac arrest and stroke. Further, the compounds of formula (I) are useful as analgesic agents, in lowering plasma free fatty acid (FFA) levels or as cardiovascular agents, e.g. treatment of myocardial ischaemia.
The compounds according to the invention are prepared as follows: ##STR5## A compound of general formula (V) may be prepared by reacting a substance of general formula (VIII) (prepared according to general method B), where B represents a hydrogen, a halogen, a pseudohalogen, an alkoxy, or a thioalkoxy group and R.sup.6 and R.sup.7 represent hydrogen or a hydroxyl protecting group such as benzoyl, p-toluyl, lower alkanoyl, an alkylated silyl group, or alternatively the two R.sup.6 may together represent a 1-methylethylidene with R.sup.7 being defined as above, with a purine derivative (II) where X and L each represents a halogen, an alkoxy or a thioalkoxy group or a (protected) amino group, giving the reaction product (III) alone or together with the corresponding .alpha.-anomer. Substitution of L in compound (III) with an alkylated amine, an alkylated hydroxylamine or a functionalised hydrazine of general formula (VI) will give compound (IV). The corresponding .alpha.-anomer of compound (III) may be reacted in a similar way. Depending upon the nature of group R.sup.6 deprotection of a compound of formula IV can be performed according to the art known (Greene, T. W., Protective Groups in Organic Synthesis, 2nd ed., 1991), to give a compound of formula V, which is a compound of formula I, ##STR6## wherein A is methyl, chloromethyl, fluoromethyl, cyanomethyl, aminomethyl, methylthiomethyl or methoxymethyl. ##STR7##
A compound of formula (VIII) where B, R.sup.6 and R.sup.7 are defined as in general method A, can be prepared from a compound (VII), where R.sup.6 and R.sup.7 are defined as in formula (I) and R.sup.5 represents a hydroxy group or a suitable leaving group such as a halogen or a halogenated sulphonate. In cases where R.sup.5 represents a hydroxy group, this can be directly alkylated to an alkoxy group with an alkylating reagent, or it can be halogenated with a suitable halogenation reagent to give compound (VIII). Alternatively, the group B may be introduced by reacting a compound (VII) where R.sup.5 represents a leaving group, with a nucleophilic reagent containing nucleophiles such as an alkoxide, thioalkoxide, or halide (incl. pseudohalides). In cases where B represents a hydrogen, this may be introduced by reduction of compound (VII) where R.sup.5 represents a hydroxyl or a suitable leaving group with a reducing reagent. The protecting groups R.sup.6 and R.sup.7 can be removed as described (Greene, T. W., Protective Groups in Organic Synthesis, 2nd ed. 1991). ##STR8##
A compound of formula (V) where B, X and R.sup.1 are defined as in general method A, can be prepared from a compound (IX) where R.sup.6 is defined as in formula (VIII) and R.sup.5 represents a hydroxy group or a suitable leaving group such as a halogen or an halogenated sulphonate. In cases where R.sup.5 represents a hydroxy group this can be directly alkylated to an alkoxy group with an alkylating reagent, or it can be halogenated with a suitable halogenation reagent to give compound (X). Alternatively, the group B may be introduced by reacting a compound (IX) where R.sup.5 represents a leaving group with a nucleophilic reagent containing nucleophiles such as an alkoxide, thioalkoxide, or halide (incl. pseudohalides).
In cases where B represents a hydrogen this may be introduced by reduction of compound (IX) where R.sup.5 represents a hydroxyl or a suitable leaving group, with a reducing reagent. The protecting groups R.sup.6 of formula (X) can be removed as described in the art known (Greene, T. W., Protective Groups in Organic Synthesis, 2nd ed 1991), to give a compound of formula V, which is a compound of formula I, ##STR9## wherein A is methyl, chloromethyl, fluoromethyl, cyanomethyl, aminomethyl, methylthiomethyl or methoxymethyl.
Methods for assessing adenosine receptor binding in vitro have been reviewed [Adenosine Receptors, Cooper, D. M. F. and Londos, C., Eds., Alan R. Liss, Inc.: New York, 1988, 43-62].
Evaluation of these compounds in established animal models has indicated that the compounds according to the invention possess desirable central nervous system properties. For example, they act as anticonvulsant agents, are effective in animal models of pain, and show cerebroprotective effects in laboratory test animals subjected to simulated cerebral ischaemia. In addition, the compounds may have efficacy as neuroprotective agents in cases of cerebral oedema and traumatic head injury.
Evaluation of in vitro binding to adenosine A.sub.1 and A.sub.2 receptors.
The affinity of the novel compounds described in this invention for the adenosine A.sub.1 receptor was determined essentially as described in the literature using [.sup.3 H]-R-PIA as a radioligand (Naunyn-Schmiedeberg's Archives of pharmacology, 1980, 313, 179-187). Affinity for the A.sub.2 receptor was measured using the radioligand [.sup.3 H]-CGS 21680 (European Journal of Pharmacology, 1989, 168, 243-246), and the values for representative compounds is given in the table below. In vitro receptor binding values obtained for the reference standards CPA [N-(cyclopentyl)adenosine] and R-PIA [(R)-N-(1-phenyl-2-propyl)adenosine]) are included for comparison. The methods both for the above in vitro examination of the compounds and the method used for DMCM-induced seizures in vivo are summarized in the European Journal of Pharmacology, 1993, 224, 221-228.
The results obtained by testing selected compounds disclosed in the present invention are shown in the table I.
TABLE I ______________________________________ DMCM-ind. Adenosine A.sub.1 Receptor A.sub.2 Receptor seizures agonist Binding Binding Ratio (ED.sub.50, tested (Ki, nM) (Ki, nM) A.sub.2 /A.sub.1 mg/kg) ______________________________________ Example 1 6.4 2739 428 0.4 Example 11 11 6600 600 4.7 Example 12 74 4655 63 6.1 Example 18 5.3 2420 457 1.0 CPA 1.2 192 77 0.2 R-PIA 1.9 116 61 0.5 ______________________________________
The compounds of the invention, together with a conventional adjuvant, carrier or diluent, and if desired in the form of a pharmaceutically acceptable acid addition salt thereof, may be placed into the form of pharmaceutical compositions and unit dosages thereof, and in such form may be employed as solids, such as tablets of filled capsules, or liquids, such as solutions, suspensions, emulsions, elixirs, or capsules filled with the same, all for oral use, in the form of suppositories for rectal administration; or in the form of sterile injectable solutions for parenteral use (including subcutaneous administration and infusion). Such pharmaceutical compositions and unit dosage forms thereof may comprise conventional ingredients in conventional proportions, with or without additional active compounds or principles, and such unit dosage forms may contain any suitable effective amount of the adenosine receptor agonist commensurate with the intended daily dosage range to be employed. Tablets containing ten (10) milligrams of active ingredient or, more broadly, ten (10) to hundred (100) milligrams, per tablet, are accordingly suitable representative unit dosage forms.
The compounds of this invention can thus be used for the formulation of pharmaceutical preparation, e.g. for oral and parenteral administration to mammals including humans, in accordance with conventional methods of galenic pharmacy.
Conventional excipients are such pharmaceutically acceptable organic or inorganic carrier substances suitable for parenteral or enteral application which do not deleteriously react with the active compounds.
Examples of such carriers are water, salt solutions, alcohols, polyethylene glycols, polyhydroxyethoxylated castor oil, gelatine, lactose amylose, magnesium stearate, talc, silicic acid, fatty acid monoglycerides and diglycerides, pentaerythritol fatty acid esters, hydroxymethylcellulose and polyvinylpyrrolidone.
The pharmaceutical preparations can be sterilized and mixed, if desired, with auxiliary agents, emulsifiers, salt for influencing osmotic pressure, buffers and/or colouring substances and the like, which do not deleteriously react with the active compounds.
For parenteral application, particularly suitable are injectable solutions or suspensions, preferably aqueous solutions with the active compound dissolved in polyhydroxylated castor oil.
Ampoules are convenient unit dosage forms.
Tablets, dragees, or capsules having talc and/or carbohydrate carrier or binder or the like, the carrier preferably being lactose and/or corn starch and/or potato starch, are particularly suitable for oral application. A syrup, elixir or the like can be used in cases where a sweetened vehicle can be employed.
Generally, the compounds of this invention are dispensed in unit form comprising 0.05-100 mg in a pharmaceutically acceptable carrier per unit dosage.
The dosage of the compounds according to this invention is 0.1-300 mg/day, preferably 10-100 mg/day, when administered to patients, e.g. humans, as a drug.
A typical tablet which may be prepared by conventional tabletting techniques contains:
Owing to activity against pain or convulsive disorders and prevention of neurodegeneration under conditions of anoxia/ischaemia the compounds of the invention are extremely useful in the treatment of related symptoms in mammals, when administered in an amount effective for agonist activity of compounds of the invention. The compounds of the invention may accordingly be administered to a subject, e.g., a living animal body, including a human, in need of adenosine receptor agonist, and if desired in the form of a pharmaceutically acceptable acid addition salt thereof (such as the hydrobromide, hydrochloride, or sulphate, in any event prepared in the usual or conventional manner, e.g., evaporation to dryness of the free base in solution together with the acid), ordinarily concurrently, simultaneously, or together with a pharmaceutically acceptable carrier or diluent, especially and preferably in the form of a pharmaceutical composition thereof, whether by oral, rectal, or parenteral (including subcutaneous) route, in an effective amount of adenosine receptor agonist, and in any event an amount which is effective for the treatment of anoxia, traumatic injury, ischemia, migraine or other pain symptoms, epilepsy, or neurodegenerative diseases owing to their adenosine receptor agonist activity. Suitable dosage ranges are 1-200 milligrams daily, 10-100 milligrams daily, and especially 30-70 milligrams daily, depending as usual upon the exact mode of administration, form in which administered, the indication toward which the administration is directed, the subject involved and the body weight of the subject involved, and the preference and experience of the physician or veterinarian in charge.